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packages/api-stack/api/api-plugin-rate-limit/src/middleware/context-augmentation.md

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+# `middleware/context-augmentation` — Hono context-variable augmentation
+
+## Purpose
+
+Augments Hono's `ContextVariableMap` interface so handlers calling
+`c.get("rateLimit")` get a typed `RateLimitDecision` rather than
+`unknown`. Without this declaration downstream handlers would have to
+narrow the value at the call site every time.
+
+The augmentation is global (Hono's `ContextVariableMap` is a single
+declaration-merged interface across the whole project), so loading
+this module via the package barrel is sufficient — consumers do not
+need to import it explicitly.
+
+## Features
+
+- Single side-effect-free module that contributes one slot to
+  `ContextVariableMap`.
+- The package barrel imports this file for its side effects so the
+  augmentation surfaces transparently.
+- The slot type is exact (`RateLimitDecision`), not the loose
+  `unknown` Hono ships by default.
+
+## Usage: Basic
+
+```ts
+import { Hono } from "hono";
+import "@suaveplan/api-plugin-rate-limit"; // side-effect import surfaces the slot
+
+const app = new Hono();
+app.get("/api/users", (c) => {
+    const decision = c.get("rateLimit"); // typed RateLimitDecision
+    return c.json({ remaining: decision.remaining });
+});
+```
+
+## Usage: Advanced
+
+```ts
+import { createMiddleware } from "hono/factory";
+import type { RateLimitDecision } from "@suaveplan/api-plugin-rate-limit";
+
+export const enrichResponseWithRateLimit = createMiddleware(async (c, next) => {
+    await next();
+    const decision: RateLimitDecision | undefined = c.get("rateLimit");
+    if (decision !== undefined) {
+        c.res.headers.set("X-Service-Quota", String(decision.remaining));
+    }
+});
+```
+
+## API Reference
+
+- Module augmentation: `declare module "hono" { interface ContextVariableMap { rateLimit: RateLimitDecision } }`.
+
+## Implementation Notes
+
+- The slot is declared without `?` so handlers may not read it before
+  the middleware has run. In practice this is fine — the middleware
+  is the only thing that writes the slot and it does so before
+  `next()`.
+- Adding additional slots requires editing this file and bumping the
+  package's minor version (the augmentation is part of the public
+  surface).
+- The augmentation file is co-located with the middleware that
+  populates the slot so future readers find both in one place.